An electromagnetic pressure reducing valve, as shown in FIG. 5 for example, comprises a solenoid 8 which causes a spool 1 to slide for connecting a control port 4 to either a supply port 5 or a tank port 6 depending on the slide position.
The spool 1 is inserted such that it is free to slide in a hole 2a in a valve housing 2. The control port 4, supply port 5 connected to an oil source P and tank port 6 connected to a tank T open into this cavity, and a feedback oil passage 3 which branches off the control port 4 enables a pressure Pc of the control port 4 to be applied to one end 1a of the spool 1.
The solenoid 8 is housed in a case 10 connected to the valve housing 2, and a plunger 9 is made to support the opposite end 1b of the spool 1 according to the magnetization state of the solenoid 1. When an electric current is passed through the solenoid 8, the solenoid 8 is magnetized, and the plunger 9 is thereby pulled toward a base 7 of the case 10 so as to push the spool 1 towards the left of the figure. The pushing force in this case is proportional to the magnetizing current flowing through the solenoid 8.
Small gaps exist between the hole 2a and the spool 1 so that the control port 4, supply port 5 and tank port 6 are interconnected. The lengths S.sub.1 and S.sub.2 of these gaps in the axial direction vary according to the position of the spool 1, however S.sub.1 +S.sub.2 is always constant.
The pressure Pc generated in the control port 4 varies according to the ratio of S.sub.1 and S.sub.2. For example, when S.sub.1 is small and S.sub.2 is large, the pressure Pc increases.
If a current i.sub.1 is supplied to the solenoid 8, a driving force F.sub.1 acts on the spool 1 tending to push it toward the left of the figure. Further, as the pressure Pc acts on the left end of the spool 1 via the feedback oil passage 3, a pushing force F.sub.2 based on this pressure Pc acts on the spool 1 in a right hand direction. If the pressure receiving surface area of the spool 1 is Aa, this pushing force is given by: EQU F.sub.2 =Pc.times.Aa (1)
The spool 1 is thus held in a position at which the driving force F.sub.1 due to the solenoid 8 and the pushing force F.sub.2 acting in the reverse direction are balanced. The driving force F.sub.1 is proportional to the electric current i.sub.1 flowing through the solenoid 8, while the pushing force F.sub.2 is proportional to the pressure Pc generated by the control port 4. In the balanced state: EQU Pc=F.sub.1 /Aa (2)
The pressure Pc is therefore proportional to the current i.sub.1 flowing through the solenoid 8. In other words, if the magnetizing current flowing through the solenoid 8 increases or decreases, the spool driving force F.sub.1 of the plunger 9 varies, so the spool is displaced to a position at which the pushing force F.sub.2 is in equilibrium with it, and the ratio of S.sub.1 to S.sub.2 varies. By varying the ratio of S.sub.1 to S.sub.2 in this manner, a high pressure supplied to the supply port 5 is reduced to an arbitrary control pressure Pc.
However, if in such an electromagnetic proportional reducing valve the current flowing through the solenoid 8 is interrupted for some reason, the spool driving force F.sub.1 of the plunger 9 drops to zero, the spool moves to the right of the figure as far as it can, and the control pressure Pc falls to the pressure Pr of the tank port 6.
If the magnetizing current i.sub.1 is interrupted due to a fault when a hydraulic actuator 20 is supporting a load due to the pressure Pc, the control port 4 is no longer to maintain the pressure, and the hydraulic actuator 20 shortens. If the actuator 20 is used in a hydraulic unit for distributing the driving force in a torque split type four-wheeled vehicle and power is no longer supplied to the solenoid 8 due to a fault in the electric circuit, therefore, four-wheeled drive becomes impossible.
To prevent this, another pressure valve could be provided for the event of an oil pressure emergency situation, but this would make the hydraulic circuit more complex and inevitably lead to greater cost.
Alternatively, the plunger 9 could be pulled toward the spool 1 by a spring so that even if the spool driving force F.sub.1 dropped to zero when the solenoid magnetizing current i.sub.1 was interrupted, the control port 4 maintained a certain minimum pressure, but in this case it would no longer be possible by ordinary means to set the pressure Pc of the control port 4 below this minimum level.